Gundog,
You can probably lookup the brochure for this particular coach. And, if you find one, it SHOULD tell you what engines and trans were available as both standard and options, based on maybe length etc. At least that's the way it is for Winnebago and Itasca products. Now, at this point, and knowing very little about the Expedition, I'm gonna guess that, because it's the model it is, I'd just about bet it's the 350HP CAT C-7. Those are good motors, in spite of some on here that think they were all bad.
If that were the case, there would be dead delivery trucks, fire trucks, cement trucks, over the road trucks and more, not to mention the ZILLION RVs that also have them, all over the country and even outside the country. Proper care of it is essential. Knowing the EXACT amount of oil during a change is imperative. And, I'd also bet that, if it's the C-7 350HP CAT, it's a 19 quart change, including the filter.
And, based on the potential fact of it having the C-7 350, I'd also bet it's got the MH3000 6-spd Allison. A great tranny. Most Fleetwood products are outfitted with a rubber roof. Do get a chance to get up on that roof and do a thorough inspection. Most of those rubber roofs hold up pretty good, for a long period of time. And if this coach as you say, has been indoors for storage for most of its life, that roof SHOULD be in outstanding shape.
Our last coach was a Fleetwood product. It was a '99 Fleetwood Bounder 34V with the V-10 and F-53 chassis. That of course, was a gas coach but, the fit and finish of that Fleetwood product was very good in my opinion. Had almost no issues with that side of things. It had Power Gear leveling jacks which, I had to rebuild two of them, no big deal. The slide (only one) was also a Power gear and, we had zero troubles with it.
The rest of the coach is standard RV suppliers. Atwood heater and water heater, Sureflo water pump, and all that. I've known a few folks with Fleetwood Discoveries and, everyone of them had absolutely great luck with them. One had a '99 with the 275 Cummins and, he put a Banks kit on it and man, was he tough to catch. The other two were an '01 and an '02, both with CAT 3126 engines which, gave them great service. Good luck and do let us know what you find.
Scott
P.S. OK, on edit: I dug up two different brochures and, I'm a bit surprised but, according to the brochure with the pertinent info I wanted, it labels the coach with the only available engine, the 300HP CAT, hmmmmm.
http://www.coltonrv.com/wp-content/uploads/2016/09/2005-Fleetwood-Expedition-Brochure-Colton.pdf